I noticed this in Opera stable (version 20) but it could potentially be the same for Chrome and Firefox.

Simply put, Flash video playback is terrible. As much as I'd like to switch to the HTML5 player in YouTube, there's a current active issue between YouTube and Opera that is blocking the HTML5 player, so the browser insists on using the Flash player.

You can force HTML5 on YouTube, e.g. the YouTube Center script. Regarding HTML5 and Opera, it probably lacks H.264/H.264+MSE support like Chromium does. As far as I know, currently Chrome and Maxthon are the only browsers that support the HTML5 video codecs (e.g. WebM and H.264) and Media Source Extensions for those codecs. Meaning you can get 480p and above support with the HTML5 player with all supported videos.